Process and device for estimating at least the vertical speed of an aircraft, in particular of a rotary wing aircraft

ABSTRACT

Process and device for estimating at least the vertical speed of an aircraft, in particular of a rotary wing aircraft.  
     The device ( 1 A) comprises a means ( 2 ) for measuring a vertical acceleration of the aircraft, a means ( 3 A) for carrying out a barometric measurement, a means ( 4 A) for forming a component from the difference between a first value incorporating the barometric measurement and a second value incorporating an estimation of the vertical speed, a means ( 5 ) for carrying out the sum of the measured vertical acceleration and of said component, and a means ( 6 ) for integrating said sum in such a way as to form the vertical speed to be estimated, and a means ( 8 A) for clipping said difference by eliminating, as appropriate, the part of said difference which is greater, in absolute value, than a predetermined value.

[0001] The present invention relates to a process and a device forestimating at least the vertical speed and possibly the altitude of anaircraft, in particular of a rotary wing aircraft.

[0002] It is known to measure the altitude and the vertical speed of anaircraft, for example of a rotary wing aircraft, with the aid ofbarometric capsules which carry out barometric measurements. However,such barometric measurements which are accurate at high altitude exhibitconsiderable errors at low altitude, especially in the case of a rotarywing aircraft, such as a helicopter for example, by virtue of theexistence of a ground effect specified hereinbelow.

[0003] It is known that the levitation of a helicopter is based on theequality between the thrust of the main rotor and the weight of thehelicopter. Normally, in particular at high altitude, the discontinuityin pressure encountered in an air jet passing through the disk of themain rotor consists essentially of a pressure reduction on the suctionsurface, this not disturbing the operation of a barometric capsuleintended for measuring the barometric altitude, whose static-pressuretaps are arranged on the fuselage of the helicopter, that is to say atthe level of the pressure surface. However, at low altitude [for exampleat an altitude of less than 18 meters (60 feet)], the flow of the airstream interacts with the ground and an overpressure is created underthe disk of the main rotor giving rise to the above-indicated groundeffect. In this case, the levitating force is of course still the same,but the new equilibrium is based almost entirely on the overpressureprevailing between the rotor disk and the ground. By virtue of theconsiderable size of a helicopter main rotor, the entire fuselage issituated in this overpressure field and hence also the static-pressuretaps of a barometric capsule, thereby giving rise to a considerableerror during the measurement of the barometric altitude by means of sucha barometric capsule. A similar error appears in the measurement, at lowaltitude, of the barometric vertical speed, that is to say of thevertical speed of the aircraft which is based on a barometricmeasurement.

[0004] In order to at least partially remedy these drawbacks, it isknown to use a value of vertical acceleration which is measured by anattitude and heading unit of the aircraft, so as to filter (and hencecorrect) the barometric altitude measurement used during the estimationof the vertical speed and/or of the altitude of an aircraft. Variousfilters formed for this purpose are known.

[0005] In a general manner, by virtue of such filters, (called “verticalloops”), the estimated vertical speed is obtained by the integration ofthe sum:

[0006] of a first component corresponding to a vertical accelerationmeasurement, and

[0007] of a second component obtained from the difference between afirst value incorporating a barometric measurement (barometric altitudeor barometric vertical speed) and a second value incorporating (directlyor after integration) a previous estimation of said vertical speed.

[0008] Although they make it possible to reduce the errors in theestimation of the vertical speed of the aircraft, such filters do notgive rise to a sufficient error reduction as to allow the use of thisestimation at low altitude. Furthermore, as the pilot uses the verticalspeed measurement as piloting reference and as the errors are spreadover time, the piloting errors stemming therefrom could be veryconsiderable and highly consequential. In particular, an underestimatedspeed would have the consequence of prompting the pilot to increase thespeed with the risk of reaching the aircraft's power limit and ofexiting the permitted flight domain.

[0009] The object of the present invention is to remedy these drawbacks.It relates to a process making it possible to provide an accurate andreliable estimation of the vertical speed of an aircraft, in particularof a rotary wing aircraft, whatever its altitude, and hence also at lowaltitude.

[0010] For this purpose, according to the invention, said processaccording to which the estimated vertical speed is obtained by theintegration of the sum:

[0011] of a first component corresponding to a vertical accelerationmeasurement, and

[0012] of a second component obtained from the difference between afirst value incorporating a barometric measurement and a second valueincorporating a previous estimation of said vertical speed,

[0013] is noteworthy in that said difference is clipped by eliminating,as appropriate, the part of said difference which is greater, inabsolute value, than a predetermined value.

[0014] Thus, by virtue of the invention, the differences which exhibithigh values are clipped. High values such as these originate essentiallyfrom said barometric measurement and represent barometric measurementerrors which are due to the aforesaid ground effect. Such errorsexhibit, as is known, short durations and very high amplitudes, whateverthe type of the aircraft and whatever its size.

[0015] Consequently, by limiting (through clipping) the effect of anybarometric measurement errors, which appear at low altitude (attouchdown and on takeoff of a rotary wing aircraft in particular), oneobtains a very accurate estimation of the vertical speed of the aircraftat low altitude and hence also over the entire flight domain, since theonly defects of accuracy of the aforesaid known process appear at lowaltitude, in particular in the case of a rotary wing aircraft at analtitude of less than 1.5 times the diameter of the main rotor of thisaircraft.

[0016] In a first embodiment, said first value is a barometric verticalspeed which is obtained by differentiation of a barometric altitudemeasurement and said second value is an estimated vertical speed.

[0017] In a second embodiment, said first value is a barometric altitudemeasurement and said second value is an altitude estimation obtained bythe integration of an estimated vertical speed.

[0018] In this case, preferably, to obtain the clipping of saiddifference, said altitude estimation is clipped.

[0019] According to the invention, the vertical speed is estimatedrepetitively and the vertical speed obtained during a previousestimation is used to form the second value used during a currentestimation.

[0020] Furthermore, in a particular embodiment, an estimation of thealtitude is moreover determined, by integrating the sum:

[0021] of a third component corresponding to said estimated verticalspeed;

[0022] of a fourth component obtained from the difference between abarometric altitude measurement and a previous estimation of saidaltitude.

[0023] Additionally, advantageously, said clipping is temporarilysuspended when one of the following conditions is satisfied:

[0024] the aircraft is in a banking phase;

[0025] the speed of the aircraft with respect to the air is greater thana predetermined speed; and

[0026] an attitude and heading unit of the aircraft is in the alignmentphase.

[0027] The present invention also relates to a device for estimating,accurately, at least the vertical speed of an aircraft, especially of arotary wing aircraft, whatever its altitude.

[0028] To do this, according to the invention, said device of the typecomprising:

[0029] a first means for measuring a vertical acceleration of theaircraft;

[0030] a second means for carrying out a barometric measurement;

[0031] a third means for forming a component from the difference betweena first value incorporating said barometric measurement and a secondvalue incorporating an estimation of the vertical speed;

[0032] a fourth means for carrying out the sum of said measured verticalacceleration and of said component; and

[0033] a fifth means for integrating said sum in such a way as to formthe vertical speed to be estimated,

[0034] is noteworthy in that it furthermore comprises a sixth means forclipping said difference by eliminating, as appropriate, the part ofsaid difference which is greater, in absolute value, than apredetermined value [for example 0.5 m/s (100 feet/minute) or 15 meters(50 feet)].

[0035] Preferably, said first means is an attitude and heading unitmounted on the aircraft.

[0036] Furthermore, in a first embodiment, said second means comprises abarometric capsule mounted on the aircraft and measuring the staticpressure so as to provide a barometric altitude measurement. In a secondembodiment, said second means comprises, mounted on the aircraft:

[0037] a barometric capsule which measures the static pressure; and

[0038] an auxiliary means which carries out a (mathematical)differentiation of this static pressure so as to provide a barometricvertical speed measurement.

[0039] Additionally, the device in accordance with the invention alsocomprises, advantageously:

[0040] a seventh means for integrating the sum of the vertical speedformed by said fifth means and of a component obtained from thedifference between, on the one hand, a barometric altitude measured bysaid second means and, on the other hand, a previous estimation of thealtitude, in such a way as to form an estimated altitude of theaircraft; and/or

[0041] an eighth means for temporarily suspending the clippingimplemented by said sixth means.

[0042] The figures of the appended drawing will elucidate the manner inwhich the invention may be embodied. In these figures, identicalreferences designate similar elements.

[0043] FIGS. 1 to 4 show the schematic diagrams of a device forestimation in accordance with the invention, respectively according tofour different embodiments.

[0044] The device in accordance with the invention and representedaccording to four different embodiments 1A, 1B, 1C and 1D respectivelyin FIGS. 1 to 4, is intended to estimate at least the vertical speedVvib of an aircraft (not represented), in particular of a rotary wingaircraft such as a helicopter.

[0045] Said device 1A, 1B, 1C, 1D is of the type comprising:

[0046] a means 2 for measuring a vertical acceleration γv of theaircraft;

[0047] a means 3A, 3B for carrying out a barometric measurement Vvb, Zb;

[0048] a means 4A, 4B, 4C, 4D for forming a component from thedifference between a first value incorporating said barometricmeasurement received from the means 3A, 3B and a second valueincorporating an estimation of the vertical speed (directly or afterintegration);

[0049] a means 5 for carrying out the sum of said vertical accelerationγv measured by the means 2 and of said component calculated by saidmeans 4A, 4B, 4C, 4D; and

[0050] a means 6 for integrating said sum (received from the means 5) insuch a way as to form the vertical speed Vvib to be estimated, which isavailable by means of a link 7.

[0051] According to the invention, said device 1A, 1B, 1C and 1Dmoreover comprises a means 8A, 8B, 8C and 8D for clipping the differenceformed by the means 4A, 4B, 4C and 4D by eliminating, as appropriate,the part of said difference which is greater, in absolute value, than apredetermined value ΔVmax, ΔZmax specified hereinbelow.

[0052] Thus, by virtue of the invention, the differences which exhibithigh values are clipped. High values such as these originate essentiallyfrom said barometric measurement Vvb, Zb (means 3A, 3B) and representbarometric measurement errors which are due to a ground effect givingrise to an overpressure between the ground and the aircraft, in the zoneof measurement of the means 3A, 3B. Such measurement errors exhibit, asis known, short durations and very high amplitudes, whatever the type ofthe aircraft and whatever its size.

[0053] Consequently, by limiting (through clipping) the effect of anybarometric measurement errors, which appear at low altitude (attouchdown and on takeoff of a rotary wing aircraft in particular), oneobtains a very accurate estimation of the vertical speed Vvib of theaircraft at low altitude and hence also over the entire flight domain,since the main defects (thus corrected) of accuracy appear essentiallyat low altitude.

[0054] Said means 2 is a standard attitude and heading unit (of the“AHRS” type: “Attitude and Heading Reference System” for example) whichis mounted on the aircraft and which, in a known manner, makes itpossible to determine a vertical acceleration γv.

[0055] In the embodiment of FIGS. 1 and 2, said means 3A provides abarometric vertical speed measurement Vvb. For this purpose, said means3A comprises, mounted on the aircraft and not represented:

[0056] a standard barometric capsule, which measures the staticpressure; and

[0057] a standard auxiliary means (pneumatic device or means ofcalculation), which carries out a (mathematical) differentiation of thisstatic pressure so as to provide a measurement of the barometricvertical speed Vvb.

[0058] Additionally, said means 4A comprises a means of calculation 9which computes the difference between the barometric vertical speed Vvbprovided by the means 3A and the vertical speed Vvib estimated during aprevious estimation.

[0059] The device 1A represented in FIG. 1 further comprises a feedbackfilter which is split into two filtering elements 10 and 11 withrespective transfer functions F1 and F2. The transfer function F1 is ofthe “low-pass” type with a much higher cutoff frequency than the overallcutoff frequency of the filter in closed loop. The clipping means 8A ismounted between these two filtering elements 10 and 11.

[0060] In this way, the unwanted components of the discrepancy signal(Vvb−Vvib) are eliminated before clipping. It is possible to choose Fland F2 in such a way that their product is equal to the standardtransfer function without means 8A, thereby guaranteeing identicaloperation in the small signals regime (that is to say in the absence ofany spike from errors when entering the ground effect or when exitingthe ground effect).

[0061] This embodiment of FIG. 1 nevertheless has a drawback due to thefact that the filtering element F1 is of “low-pass” type. Specifically,in the presence of error spikes, the output state has altered in anonzero time (after-effect), beyond the clipping threshold.Consequently, the return of the filter to the linear regime (that is tosay, to the state where the clipping is no longer relevant) will take acertain time, this having the effect of increasing the weight of theerror which is injected into the loop.

[0062] The embodiment 1B of FIG. 2 makes it possible to remedy thisdrawback.

[0063] To do this, the means 8B is integrated directly into the element10 to form a feedback filtering element 12. The means 8A will limit thefluctuation in the output state variable of this feedback filter so thatthe weight of the injected error is much lower than in the embodiment ofFIG. 1A. The speed difference (Vvib−Vvb) is limited to ±ΔVmax, ΔVmaxbeing for example equal to 100 feet/minute (0.5 m/s).

[0064] Additionally, in the embodiments of FIGS. 3 and 4, the means 3Bprovides a barometric altitude measurement Zb. For this purpose, thismeans 3B comprises a standard barometric capsule (not represented),mounted on the aircraft and measuring the static pressure.

[0065] Furthermore, the means 4C, 4D comprises a means of calculation 13which computes the difference between the barometric altitude Zbmeasured by the means 3B and an estimated altitude value Zibincorporating the estimated vertical speed Vvib and specifiedhereinbelow.

[0066] In these embodiments, the means 8C, 8D does not carry out theclipping of a vertical speed difference, but the clipping of an altitudedifference emanating from the means of calculation 13.

[0067] The device 1C furthermore comprises a filtering element 14 whichis mounted between said means 8C and the summator 5.

[0068] Said device 1C provides, in addition to a vertical speedestimation Vvib, an altitude estimation Zib which is available by meansof a link 15.

[0069] This estimated altitude Zib is obtained with the aid of a means16 which comprises an integrator 17 which integrates the result of a sumprovided by a summator 18. This summator 18 computes the sum of anestimation of the vertical speed Vvib provided by the integrator 6 andof a component representing the aforesaid difference emanating from themeans of calculation 13 and which is filtered by a filtering element 19.

[0070] The two filtering elements 14 and 19 may be of any order. Theorder of the overall filter corresponds to the sum, increased by 2, ofthe orders of the two corresponding transfer functions. By acting onthese filtering elements 14 and 19 it is possible to optimize theselectivity between the vertical acceleration and the barometricaltitude.

[0071] Additionally, the embodiment 1D of FIG. 4 is similar to that 1Cof FIG. 3 and comprises filtering elements 20 and 21 which are of thesame type as the filtering elements 14 and 19 of FIG. 3. However, theobject of the means 8D is to directly limit the fluctuation of theintegrator 17, rather than to carry out the clipping of the signalemanating from the means of calculation 13. Thus, the altitude Zibemanating from the integrator 17 is limited to an altitude ±ΔZmax, ΔZmaxbeing equal for example to 50 feet (15 meters).

[0072] Additionally, according to the invention, said device 1A, 1B, 1Cand 1D moreover comprises a means 24 for temporarily suspending theclipping implemented by the means 8A, 8B, 8C and 8D, as is illustratedby a chain-dotted link 25A, 25B, 25C and 25D. In a preferred embodiment,said means 24 temporarily suspends said clipping, when one of thefollowing conditions is satisfied:

[0073] the aircraft is in a banking phase (with maintenance of thetemporary suspension of the clipping for a predefined duration after areturn to horizontal flight);

[0074] the speed of the aircraft with respect to the air is greater thana predetermined speed; and

[0075] an attitude and heading unit (not represented) of the aircraft isin the alignment phase.

[0076] It will be noted that this means 24 makes it possible toeliminate a negative secondary effect of the clipping implemented by themeans 8A, 8B, 8C and 8D in accordance with the invention. This secondaryeffect (of the clipping which is envisaged in the feedback loop), is theconsiderable increasing of the duration of estimation of a new bias ofvertical acceleration, in the event of a fast and large-amplitudevariation of said bias.

1. A process for estimating at least the vertical speed of an aircraft,in particular of a rotary wing aircraft, according to which process theestimated vertical speed is obtained by the integration of the sum: of afirst component corresponding to a vertical acceleration measurement,and of a second component obtained from the difference between a firstvalue incorporating a barometric measurement and a second valueincorporating a previous estimation of said vertical speed, wherein saiddifference is clipped by eliminating, as appropriate, the part of saiddifference which is greater, in absolute value, than a predeterminedvalue (ΔVmax, ΔZmax).
 2. The process as claimed in claim 1, wherein saidfirst value is a barometric vertical speed which is obtained bydifferentiation of a barometric altitude measurement and said secondvalue is an estimated vertical speed.
 3. The process as claimed in claim1, wherein said first value is a barometric altitude measurement andsaid second value is an altitude estimation obtained by the integrationof an estimated vertical speed.
 4. The process as claimed in claim 3,wherein, to obtain the clipping of said difference, said altitudeestimation is clipped.
 5. The process as claimed in claim 1, wherein thevertical speed is estimated repetitively and wherein the vertical speedobtained during a previous estimation is used to form the second valueused during a current estimation.
 6. The process as claimed in claim 1,wherein an estimation of the altitude is moreover determined, byintegrating the sum: of a third component corresponding to saidestimated vertical speed; of a fourth component obtained from thedifference between a barometric altitude measurement and a previousestimation of said altitude.
 7. The process as claimed in claim 1,wherein said clipping is temporarily suspended when one of the followingconditions is satisfied: the aircraft is in a banking phase; the speedof the aircraft with respect to the air is greater than a predeterminedspeed; and an attitude and heading unit of the aircraft is in thealignment phase.
 8. A device for estimating at least the vertical speedof an aircraft, in particular of a rotary wing aircraft, said device(1A, 1B, 1C, 1D) comprising: a first means (2) for measuring a verticalacceleration of the aircraft; a second means (3A, 3B) for carrying out abarometric measurement; a third means (4A, 4B, 4C, 4D) for forming acomponent from the difference between a first value incorporating saidbarometric measurement and a second value incorporating an estimation ofthe vertical speed; a fourth means (5) for carrying out the sum of saidmeasured vertical acceleration and of said component; and a fifth means(6) for integrating said sum in such a way as to form the vertical speedto be estimated, wherein it furthermore comprises a sixth means (8A, 8B,8C, 8D) for clipping said difference by eliminating, as appropriate, thepart of said difference which is greater, in absolute value, than apredetermined value (ΔVmax, ΔZmax).
 9. The device as claimed in claim 8,wherein said first means (2) is an attitude and heading unit mounted onthe aircraft.
 10. The device as claimed in claim 8, wherein said secondmeans (3B) comprises a barometric capsule mounted on the aircraft andmeasuring the static pressure so as to provide a barometric altitudemeasurement.
 11. The device as claimed in claim 8, wherein said secondmeans (3A) comprises, mounted on the aircraft: a barometric capsulewhich measures the static pressure; and an auxiliary means which carriesout a differentiation of this static pressure so as to provide abarometric vertical speed measurement.
 12. The device as claimed inclaim 8, wherein it furthermore comprises a seventh means (16) forintegrating the sum of the vertical speed formed by said fifth means (6)and of a component obtained from the difference between, on the onehand, a barometric altitude measured by said second means (3B) and, onthe other hand, a previous estimation of the altitude, in such a way asto form an estimated altitude of the aircraft.
 13. The device as claimedin claim 8, wherein it furthermore comprises an eighth means (24) fortemporarily suspending the clipping implemented by said sixth means (8A,8B, 8C, 8D).